In recent years, in electronic and optical elements such as portable terminal devices such as cellular phones and PDA, digital cameras, digital video cameras, reduction in thickness and weight, and furthermore, production of paper-like display parts have been attempted in parallel with enhanced functionality. Tapes for supplying pressure sensitive adhesive layers have been used as materials for the attachment of components of these electronic and optical devices. However, the pressure sensitive adhesive layer of such a tape is insufficient in adhesive strength and, when exposed to high temperatures, suffers from, for example, significantly reduced adhesive strength.
A liquid crystal panel and other components are disposed in the image display part of an image display apparatus. The liquid crystal panel includes, for example, a liquid crystal cell and polarizing plates laminated to both sides of the liquid crystal cell, and is very weak to impacts from the outside. For this reason, in conventional image display apparatuses, a transparent protective member is arranged at the front of the liquid crystal panel with a certain gap in between. The gap has a role of buffering the impact from the outside for protecting the display panel. However, since the refractive index of the protective member greatly differs from the refractive index of a gap (air space), Fresnel reflection occurs in the interface between the protective member and the gap. This disadvantageously causes a reduction in the visibility of the image displayed on the liquid crystal panel.
To counter the foregoing problem, it has been contemplated in the art to dispose a transparent adhesive film between the liquid crystal panel and the protective member to improve the visibility of an image (e.g., PTL 1). However, if there are irregularities on the surface of one or both of the liquid crystal panel and protective member, the adhesive film cannot sufficiently follow the irregularities easily generating gaps in the vicinity thereof. The gaps disadvantageously cause a further reduction in the visibility. This problem easily occurs particularly in a touch panel-type image display apparatus in which electrodes are formed on the protective member.
It has also been contemplated in the art to dispose an acrylic resin layer having high transparency between the liquid crystal panel and the protective member (PTL 2). However, since the refractive index of a transparent resin essentially including a conventional acrylic resin is lower than the refractive index of the protective member, the visibility of the image display apparatus is not yet sufficient. Further, in order to increase the refractive index of the transparent resin layer with which the space between the image panel and the protective member is filled, the addition of a terpene resin having a high refractive index to the acrylic resin has also been studied. However, disadvantageously, a photo-cured product of these resins is cloudy or has reduced adhesive strength between the image panel and the protective member.
Polyisopropyltoluene is known as a tackifier, and the addition of about 10 parts by weight thereof to 100 parts by weight of a polymer for adhesion is common (PTLs 3 to 6).
Citation ListPatent LiteraturePTL 1Japanese Patent Application Laid-Open No. 2007-169327PTL 2Japanese Patent Application Laid-Open No. 2005-134841PTL 3Japanese Patent Application Laid-Open No. 2011-140594PTL 4Japanese Patent Application Laid-Open No. 2000-169819PTL 5Japanese Patent Application Laid-Open No. 2000-273428PTL 6Japanese Patent Application Laid-Open No. 2003-277709